This relates to improved bridge circuits. Because the use of such circuits with strain gauges is of primary interest, the invention will be described in terms of such application. It will be recognized, however, that the principles disclosed may have application to other circuits.
A strain gauge is typically used by bonding it to a flexible object and measuring the change in voltage across the gauge or the change in gauge resistance as different loads are applied to the object. It is particularly advantageous to use a Wheatstone bridge in which two strain gauges are connected in series on one side of the bridge and two resistors are connected in series on the other side. Each of these four elements is in a separate diagonal of the bridge with the supply voltage applied to the nodes between the two sides of the bridge and the output voltage measured between the node between the two resistors and the node between the two strain gauges. Since the function of the two resistors is to provide a reference voltage at the node between them, their side of the bridge will be referred to as the reference side. If the gauges are mounted on opposite sides of the object so that bending of the object applies a tensile loading to one gauge and a compressive loading to the other, the changes in resistance of the gauges tend to be equal in magnitude but opposite in polarity. For these conditions, the ratio of the resistances of the two strain gauges is a function of the amount of deflection in the object. Hence, the output voltage can be related to the amount of deflection in the object.
Recent improvements in the art have led to increasing use of semiconductor strain gauges. As is well known, such strain gauges offer significant advantages over prior art foil or wire strain gauges since the sensitivity of the semiconductor gauges is hundreds of times greater than that of typical metallic gauges. However, semiconductor strain gauges have both a large temperature coefficient of resistance and a large temperature coefficient of gauge factor or sensitivity. Thus, both their resistance and their rate of change of resistance with applied stress vary appreciably with temperature. Semiconductor strain gauges can be made so that these temperature coefficients in different devices are approximately the same. However, when the gauges are bonded to an object, certain uncontrollable temperature induced strains are created that modify the temperature coefficients of the gauges. As a result, the voltage output from the bridge is a function of temperature.
Typically, this variation in output voltage because of changes in resistance with temperature is compensated by measuring the resistance of the gauges under zero stress at two temperatures and selecting a series/parallel network of resistance for one gauge which offsets the effects of its temperature coefficient of resistance enough that the ratio of the resistances in the two strain gauge diagonals at the two compensation temperatures is identical. This process is called temperature compensation. While this temperture compensation does improve the performance of the circuit as a measuring device, it does not guarantee that the resistance ratios are the same at any other temperature because of the complex effects of the temperature induced strain in the gauges. Moreover, no correction is made by this temperature compensation process for the variation in output voltage because of change in sensitivity with temperature.
The variation in output voltage because of change in sensitivity with temperature is compensated by introducing a resistor in series or parallel with the bridge. The value of this resistor is selected to balance the temperature coefficient of sensitivity. More particularly, once the bridge is temperature compensated at its two compensation temperatures, its output voltage is measured at these two temperatures with maximum deflection being applied to the object on which the gauges are mounted. A series or paralel resistor is then selected so that the output voltage under this condition is the same at both compensation temperatures. This process is called span compensation.
The above referenced patent application of Howard R. Branch, III, describes a method and circuit for achieving improved temperature compensation at three different temperatures. Thus, in a bridge circuit according to the Branch invention the ratio of the resistances in the two strain gauge diagonals is substantially the same at three different temperatures. This is accomplished by incorporating into the strain gauge side of the bridge both the span compensation resistor and the series/parallel network of temperature compensation resistors. While this technique does produce improved temperature compensation, span compensation is still only achieved at two temperatures. Between these two temperatures there typically is a non-zero span error.
It frequently is also desirable to suppress the zero output level from the bridge circuit. For example, it is common practice to use a two wire circuit both to supply power to the bridge circuit and to produce an output signal proportional to the output voltage of the bridge circuit. In such a case the output signal might vary from four to twenty milliamps. When the bridge circuit is used to measure pressure differential, the four milliamp output signal might correspond to zero pressure differential while the twenty milliamp output might correspond to a maximum pressure differential such as 200 pounds per square inch. If, however, the pressure range between 100 and 200 pounds per square inch is the only range of interest, it is desirable to modify the circuit output so that a four milliamp output corresponds to 100 pounds pressure differential instead of zero pressure differential. However, prior art techniques for making such modifications typically upset the resistance relations in the bridge which necessarily would destroy the temperature and span compensation provided for in the aforementioned application of Howard R. Branch, III.
In some applications it may also be desirable to provide an output signal which is not a linear function of the output voltage across the bridge. For example, it frequently is advantageous to provide an output signal which is the square root of the pressure differential that might be measured by a pair of strain gauges because the flow of fluid in a pipe is proportional to the square root of the pressure differential. However, the provision of such a function in the prior art typically requires relatively complicated circuitry.
I have devised circuits which provide for improved span compensation, scale adjustment, and non-linear output functions while being compatible with the temperature and span compensation described in the Branch application. Each circuit comprises a bridge circuit in one arm of which are first and second series connected electrical elements whose resistance varies as a function of at least two variables and in the other arm of which are third, fourth and fifth series connected electrical elements whose resistance is substantially constant with respect to said two variables under operating conditions. Illustratively, the first of the two variables is temperature and the second is stress.
To reduce span compensation errors, the circuit further comprises sixth and seventh series connected electrical elements which are connected between the input nodes of the bridge circuit, one of which elements has a positive temperature coefficient of resistance and the other of which has a negative coefficient of resistance with respect to a first one of said two variables. A first terminal of a voltage supply is connected to a first input node of the bridge circuit. A parallel combination of eighth and ninth electrical elements is connected between a second input node of the bridge circuit and a second terminal of the voltage supply. One of these two electrical elements has a positive temperature coefficient of resistance and the other a negative temperature coefficient of resistance with respect to said first variable. The sixth, seventh, eighth and ninth electrical elements are selected as detailed below to reduce span compensation errors at a third value of the first variable.
Each circuit further comprises output means, such as a high gain differential amplifier, having first and second input terminals, one of which is connected to a first output node of the bridge circuit located between said first and second electrical elements and the other of which is connected to a second output node of the bridge circuit located between said fourth and fifth electrical elements. Preferably, my circuit is used in a two wire system in which power is supplied to the circuit over the same two wires over which the output signal is transmitted. In such a system the output means is used to control current flow in said wires.
My invention also comprises a constant current source which is connected between the first output of the bridge circuit and a node between the third and fourth electrical elements. By varying the current from this source one can adjust the voltage at the output node in the reference arm of the bridge so that the minimum output signal does not correspond to the zero level in the variable being sensed by the bridge circuit. In contrast to prior art techniques of adjusting this voltage by changing the resistance values in the reference arm, the use of a constant current source facilitates the achievement of temperature and span compensation.
Alternatively, the constant current source may be used to calibrate a non-linear circuit. In this application a function generator and a feedback resistor are connected between an output terminal of the circuit and an input terminal to the differential amplifier. For this configuration the output signal from the differential amplifier is proportional to that function of the input signal which is the inverse of the function produced by the function generator. For example, the output of the differential amplifier is proportional to the square root of the input signal if the function generator is a squaring circuit. This type of function generator typically has a non-zero output at a point which corresponds to the zero level in the variable being sensed by the bridge circuit. Accordingly, it is necessary to compensate for this output by modifying the voltage applied to the differential amplifier. I have found that this may readily be accomplished by use of the constant current source in a similr fashion to its use in a linear circuit.